Preparation,Functionalization And Stability Test Of Rigid Covalent Organic Frameworks

Constructed with pure organic building blocks via dynamic covalent bonds,covalent organic frameworks(COFs)demonstrate abundant channels,high specific surface areas and rich functional moieties.Due to those specific properties,COFs are widely inspired in the field such as gas storage,heterogeneous catalysis,fluorescence and energy storage,et al.As those flexible and fragile coordination bonds are replaced with dynamic covalent bonds,COFs with high structure designability and promising chemical stability seem more promising for practical applications than the extensively studied MOFs.For common COFs linkages,especially imine and its derivates,however,the chemical stabilities are often not enough in strong acid/base or in long-run use.The enhancement of chemical stability is crucial for further development and practical applications of these materials.Following are some advantages for developing robust COFs:(1)Highly stable COFs are functional in more fields,especially in some harsh conditions,such as waste gas adsorption,sewage treatment and heterogeneous catalysis under intense conditions.(2)Some severe post-modification conditions are applicable for rigid frameworks.Such various functional moieties could be introduced into the frameworks and anchored tightly.(3)The utilization of robust COFs could improve cycling stability or working life conspicuously,thus reducing the application cost.Based on the above analysis,we made some efforts in design,functionalization and characterization of rigid COFs.There are three mean contents as follows:(1)We designed and prepared a series of chemically stable polyarylether-based covalent organic frameworks(PAE-COFs),including microporous JUC-505 and mesoporous JUC-506.The frameworks were completely fused by phenyl rings and dioxane rings,leading to rigidity and chemical stability.The stabilities were characterized by powder X-ray diffraction(PXRD)and Fourier transform infrared(FTIR)spectra.PAE-COFs could suffer all experiment conditions(including common organic solvents,boiling water,strong acid,base,oxidant and reductant)with almost unchanged PXRD patterns and FTIR spectrums.(2)Based on the ultrahigh chemical stability of JUC-505,we carried out post-modification with some harsh conditions,such as refluxing in Na OH solution(for JUC-505-COOH)or Li Al H₄solution(for JUC-505-NH₂).Afterwards,these functionalized PAE-COFs were utilized for antibiotics removal from aqueous solutions with different p H.Both JUC-505-COOH and JUC-505-NH₂demonstrated much higher uptakes than unmodified JUC-505.Moreover,functionalized PAE-COFs were robust with almost unchanged uptake performance after 6 crycles.(3)We prepared ketoenamine-based KEA-COFs,including JUC-520 and JUC-525.Both KEA-COFs were rigid in common organic solvents or acid/basic solutions for at least 7 days with remained or partially remained PXRD patterns.Moreover,we tried to develop a quantitative method for analyzing the change of crystallinity after stability test.Based on the PXRD patterns analyzing of repeated scan and repeated test,we found that I_4.6reland A_4.6relwere believable and suitable parameters for quantitative analysis.I_4.6reland A_4.6relwere identified as the relative peak intensity(ratio of net peak intensity to total peak intensity)or the relative peak area(ratio of net peak area to total peak area)of the mean diffraction peak(2theta?4.6°for JUC-520.Both I_4.6reland A_4.6relexhibited positive relations with the crystallization of JUC-520,held steady in the measurement(variable coefficient was less than 5%within six repeated scans)and were repeatable(variable coefficient was less than 5%among six parallel tests).As the result,we believed that were convincing parameters for quantitative analyzing the crystallinity of JUC-520.By dividing relative peak intensity(I_4.6rel)or relative peak area(A_4.6rel)after stability test over relative peak intensity(I⁰_4.6rel)or relative peak area(A⁰_4.6rel)before stability test,we got two parameters I_4.6rel'and A_4.6rel',respectivity,which were coincide and represented the crystallinity remaining after stability test.